The invention relates to a ruthenium electrolyte which is suitable for depositing decorative and technical layers having particular blackness. Furthermore, the invention relates to a method for depositing decorative and technical layers of ruthenium having particular blackness (“black ruthenium”) on pieces of jewelry, decorative goods, consumer goods and technical articles.
Consumer goods and technical articles, pieces of jewelry and decorative goods are finished with thin oxidation-stable metal layers for protection from corrosion and/or for visual upgrading. These layers must be mechanically stable and should show no tarnishing or signs of wear even with relatively long use. A tried and tested means of producing such layers comprises electroplating methods by means of which a multiplicity of metal and alloy layers can be obtained in high quality. Examples well known from everyday life are electrodeposited bronze and brass layers on door handles or knobs, chromium coatings on vehicle parts, galvanized tools or gold coatings on watchstraps.
A particular challenge in the area of electrochemical finishing is the production of oxidation-stable and mechanically strong black metal layers which may be of interest not only in the area of decoration and jewelry but also for technical applications, for example in the area of solar engineering. Only a few metals are available for producing oxidation-stable, black layers. In addition to ruthenium, rhodium and nickel are suitable. The use of the noble metal rhodium is limited to the area of jewelry, owing to the high raw material costs. The use of economical nickel and nickel-containing alloys is possible, particularly in the area of jewelry and consumer goods, only in exceptional cases and taking into account stringent requirements, since nickel and nickel-containing metal layers are contact allergens. The use of ruthenium is an expedient alternative for all fields of use described.
Electrolytes for the production of black ruthenium layers in electroplating methods for finishing are known in the prior art. The most customary baths contain ruthenium in a form complexed with amidosulfonic acid or ruthenium as a nitridochloro or nitridobromo complex.
For example JP 63259095 describes a method for ruthenium electroplating using a bath containing 5 g/l of ruthenium and from 100 to 150 g/l of amidosulfonic acid. WO 2001/011113 discloses a ruthenium electrolyte which contains ruthenium sulfate and sulfamic acid (amidosulfonic acid). A thio compound is used as a blackening additive. For protecting the thio compound from decomposition by anodic oxidation, a sacrificial substance must also be added. An electrolyte for electrochemical deposition of low-stress ruthenium layers having good tensile strength according to DE 197 41 990 contains ruthenium in a form complexed with amidosulfuric acid and pyridine or N-alkylated pyridinium salts. U.S. Pat. No. 4,375,392 claims an acidic electrolyte for the deposition of ruthenium onto various substrates containing a complex of ruthenium and amidosulfonic acid, which is present in a molar concentration of from 4 to 10 mol of amidosulfonic acid per mole of ruthenium and in suitable concentration, and containing a second compound of a metal selected from the group consisting of nickel, cobalt, iron, tin, lead and magnesium. The concentration of the second metal is chosen so that ruthenium layers having good tensile strength can be deposited. The pH of the bath is from 0.1 to 2.2.
DE 1 959 907 describes the use of the dinuclear ruthenium complex [Ru2NClxBr8-x(H2O)2]3- in an electroplating bath. In one embodiment, the nitridochloro complex [Ru2NCl8(H2O)2]3- is used. This nitridochloro complex of ruthenium is also used in the aqueous, nonacidic bath for the electrodeposition of ruthenium, which is described in U.S. Pat. No. 4,297,178. Oxalic acid or an oxalate is also present therein.
JP 56119791 has a ruthenium electrolyte as the subject of the invention, which, in addition to from 1 to 20 g/l of ruthenium, contains one or more compounds selected from the group of di- and tricarboxylic acids, benzenesulfonic acid, N-containing aromatics and amino acids or derivatives of said compounds and in which moreover from 0.01 to 10 g/l of a thio compound are present as a blackening additive.
JP 2054792 preferably contains ruthenium sulfate, an inorganic acid, preferably sulfuric acid, and a “metal of group III”, preferably Sc, Y, In or Ga, in addition to an inorganic ruthenium salt.
For the finishing of jewelry and decorative goods, black layers must have not only excellent mechanical adhesive strength but also a satisfactory visual quality. If required, they must be capable of being produced in bright or dull form and with very deep blackness. The same applies to applications in the technical area, in particular in solar engineering. Black layers for the finishing of consumer goods must moreover meet high requirements with regard to the mechanical stability. In particular, they must not exhibit any black abrasion even with frequent use over a relatively long time.
The ruthenium baths which are described in the prior art and meet these requirements are either dependent on the use of toxicologically unsafe compounds, such as thio compounds, as a blackening additive or contain a further transition metal for ensuring the required mechanical adhesive strength, which complicates the handling of the bath during the deposition process.